Ranging in a communication system means that a mobile station adjusts transmission parameters (frequency offset, time offset, transmission power, and the like) for uplink communication with a base station when an initial network registration procedure is performed or if necessary to continue such uplink communication with the base station even after the network registration procedure is performed.
A wireless metropolitan area network (Wireless MAN) system according to the IEEE 802.16 can perform ranging of two types below. The first type is initial ranging corresponding to the case where a mobile station (MS) performs synchronization with a new base, station (BS) to connect with the base station in such a way to be first powered on or perform handover. The second type is periodic ranging corresponding to the case where a base station periodically performs synchronization with the mobile station at the request of the mobile station or even at no request of the mobile station when readjustment of synchronization with the mobile station is needed.
First, the procedure of the initial ranging will be described below.
1) The mobile station receives uplink transmission parameters after acquiring downlink synchronization.
2) The mobile station selects one of slots of ranging subchannels allocated within a frame structure and forwards ranging request message (RNG-REQ) to the base station using an initial ranging code.
3) The base station receives the ranging request message and forwards all of required transmission parameter adjustment values and status information to the mobile station through ranging response message (RNG-RSP).
4) If the status information indicates ‘success,’ the base station allocates uplink bandwidth for CDMA Allocation IE required to allow the mobile station to transmit the ranging request message (RNG-REQ). If the status information indicates ‘continue,’ the mobile station returns to the step 2) and repeats the steps until the status information indicates ‘ success.’
Next, the procedure of the periodic ranging will be described below.
1) The mobile station selects a periodic ranging slot and a periodic ranging code and transmits the ranging code through the selected ranging slot.
2) The base station broadcasts the received ranging code and the ranging slot through the ranging response message (RNG-RSP). The ranging response message (RNG-RSP) includes status information and information (time, power, and frequency correction) required for adjustment.
3) If the status information indicates ‘continue,’ the mobile station reselects ranging slot and ranging code and repeats the steps 1) and 2).
4) In ranging method of an orthogonal frequency division multiple access (OFDMA) system, a timer for periodic ranging is controlled by the mobile station.
In case of the periodic ranging, the base station determines the channel status with the mobile station, and if it is determined that the ranging is required, the base station can request the ranging through an unsolicited ranging response message (RNG-RSP) even in the case that the ranging request message (RNG-REQ) is not requested from the base station.
Hereinafter, the ranging operation performed during initialization of the mobile station in a single carrier and orthogonal frequency division multiple (OFDM) mode of a wireless metropolitan area network (Wireless MAN) system will be described.
The base station allocates a contention based initial ranging field to the mobile stations through an uplink map. Each of the mobile stations forwards the ranging request message which includes its MAC address to the base station using the ranging field of the uplink. If the ranging request message received in the base station cannot be decoded due to collision with another ranging request message transmitted from another mobile station, the base station forwards ranging response message which includes the ranging field and a frame number received through the ranging request message along with the uplink transmission parameter adjustment values.
If the ranging request message received in the base station can be decoded, the base station forwards the uplink transmission parameter adjustment values of the mobile station through the ranging response message. If the uplink transmission parameters of the mobile station are successfully adjusted, the base station forwards ranging response message to the mobile station, wherein the ranging response message includes a basic management connection identifier (CID) and a primary management connection identifier (CID). The base station allocates an uplink band through the uplink map to allow the mobile station to transmit the ranging request message. At this time, the base station allocates a non-contention based uplink band through the basic management connection identifier of the mobile station. The mobile station which has been allocated with the uplink band transmits the ranging request message, while the base station transmits the ranging response message in response to the ranging request message of the mobile station. In this case, coding and modulation modes of a downlink burst between the mobile station and the base station may be adjusted.
Hereinafter, the ranging operation performed during initialization of the mobile station in an OFDMA wireless metropolitan area network (Wireless MAN) system will be described.
The OFDMA wireless MAN system performs an uplink bandwidth request and ranging request using CDMA codes to allow the mobile station to adjust uplink transmission parameters. The base station forwards CDMA code sets for ranging and bandwidth request to the mobile stations in a broadcast type through an uplink channel descriptor (UCD) message. The mobile station optionally selects a suitable ranging code from the CDMA codes acquired from the UCD message and transmits the selected ranging code through the uplink field allocated for ranging. Table 1 and Table 2 respectively illustrate the UCD message and examples of TLV parameters related to the ranging and bandwidth request included in the UCD message.
TABLE 1SyntaxSizeNotesUCD_Message_Format( ) {Management Message Type = 08 bitsConfiguration Change Count8 bitsranging Backoff Start8 bitsranging Backoff End8 bitsRequest Backoff Start8 bitsRequest Backoff End8 bitsTLV Encoded information for thevariableTLV specificoverall channelBegin PHY Specific Section {See applicable PHYsection.for(i= 1; i <= n; i++) {For each uplink burstprofile 1 to n.Uplink_Burst_ProfilevariablePHY specific}}}
TABLE 2TypeName(1 byte)LengthValueInitial ranging1501Number of initial ranging CDMA codes. PossibleCodesvalues are 0-255.aPeriodic1511Number of periodic ranging CDMA codes. Possibleranging codesvalues are 0-255.aHandover1941Number of handover ranging CDMA codes. Possibleranging codesvalues are 0-255.aBandwidth1521Number of bandwidth request codes. Possible valuesrequest codesare 0-255.aPeriodic1531Initial backoff window size for periodic rangingranging backoffcontention, expressed as a power of 2. Range: 0-15start(the highest order bits shall be unused and set to 0).Periodic1541Final backoff window size for periodic rangingranging backoffcontention, expressed as a power of 2. Range: 0-15end(the highest order bits shall be unused and set to 0).Start of ranging1551Indicates the starting number, S, of the group ofcodes groupcodes used for this uplink. All the ranging codes usedon this uplink will be between S and ((S + N + M + L + O)mod 256). Where, N is the number of initial-rangingcodes. M is the number of periodic-ranging codes. Lis the number of bandwidth-request codes. O is thenumber of initial-ranging codes. M is the number ofhandover-ranging codes. The range of values is 0 S ≦≦255
The base station allocates a contention based ranging field to the mobile stations through an uplink map information element included in an uplink map. Each of the mobile stations allocates the ranging field in such a way to divide it into initial ranging codes, handover ranging codes, periodic ranging codes, and bandwidth request codes depending on ranging service.
The base station which has received the ranging codes from the mobile station sets transmission power adjustment values, time and frequency adjustment values, ranging status information (success and fail), and the like, which are required for uplink transmission synchronization of the mobile station, through ranging response message (RNG-RSP) as shown in Table 3 and forwards them.
TABLE 3SyntaxSizeNotesRNG-RSP_Message_Format( ) {Management Message Type = 58 bitsUplink Channel ID8 bitsTLV Encoded InformationvariableTLV specific}
Table 4 illustrates examples of TLV parameters included in the ranging response message.
TABLE 4TypeName(1 byte)LengthValue (variable-length)Timing Adjust14Tx timing offset adjustment (signed 32-bit).The time required to advance SS transmissionso frames arrive at the expected time instanceat the BS. Units are PHY specific (see 10.3).Power Level21Tx Power offset adjustment (signed 8-bit, 0.25AdjustdB units)Specifies the relative change in transmissionpower level that the SS is to make in orderthat transmissions arrive at the BS at thedesired power. When subchannelization isemployed, The subscriber shall interpret thepower offset adjustment as a required changeto the transmitted power density.Offset34Tx frequency offset adjustment (signed 32-bit, Hz units)FrequencySpecifies the relative change in transmissionAdjustfrequency that the SS is to make in order tobetter match the BS. (This is fine-frequencyadjustment within a channel, not reassignmentto a different channel.)Ranging Status41Used to indicate whether uplink messages arereceived within acceptable limits by BS.1 = continue, 2 = abort, 3 = success, 4 = re-rangeRanging code1504Bits 31:22 - Used to indicate the OFDM timeattributessymbol reference that was used to transmit theranging code.Bits 21:16 - Used to indicate the OFDMAsubchannel reference that was used to transmit theranging code.Bits 15:8 - Used to indicate the ranging codeindex that was sent by the SS.Bits 7:0 - The 8 least significant bits of theframe number of the OFDMA frame wherethe SS sent the ranging code.Downlink72This parameter is sent in response to theOperationalRNG-REQ Requested Downlink Burst ProfileBurst Profileparameter.Byte 0: Specifies the least robust DIUC thatmay be used by the BS for transmissions totheSS.Byte 1: Configuration Change Count value ofDCD defining the burst profile associatedwith DIUC.SS MAC86SS MAC Address in MAC-48 formatAddressBasic CID92Basic CID assigned by BS at initial access.Primary102Primary Management CID assigned by BS atManagementinitial access.CID
If it is determined that the uplink transmission parameters have been successfully adjusted, the base station sets the ranging status to ‘success’ and allocates an uplink bandwidth to a corresponding mobile station through an uplink information element (CDMA_Allocation_IE) as shown in Table 5, thereby allowing the mobile station to transmit the ranging request message.
TABLE 5SyntaxSizeNotesCDMA_Allocation_IE( ) {Duration6 bitsRepetition Coding Indication2 bits0b00 - No repetition coding0b01 - Repetition coding of 2 used0b10 - Repetition coding of 4 used0b11 - Repetition coding of 6 usedRanging Code8 bitsRanging Symbol8 bitsRanging subchannel7 bitsBW request mandatory1 bit1 = yes, 0 = no}
The mobile station which has been allocated with the uplink field transmits ranging request message (RNG-REQ) as shown in Table 6 to the base station along with its identifier (MAC address). The base station which has received the ranging request message allocates management CIDs to the corresponding mobile station in such a way to forward the ranging response message, which includes basic management CID and primary management CID as shown in Table 8, to the corresponding mobile station.
TABLE 6SyntaxSizeNotesRNG-REQ_Message_Format( ) {Management Message Type = 48 bitsDownlink Channel ID8 bitsTLV Encoded InformationvariableTLV specific}
Table 7 and Table 8 illustrate examples of TLV parameters included in the ranging request message of Table 6.
TABLE 7Type(1Namebyte)LengthValue (variable-length)Requested11Bits 0-3: DIUC of the downlink burst profileDownlinkrequested by the SS for downlink traffic. BitsBurst4-7: 4 LSB of Configuration Change CountProfilevalue of DCD defining the burst profileassociated with DIUC.SS MAC26The MAC address of the SSAddressRanging31A parameter indicating a potential errorAnomaliescondition detected by the SS during theranging process. Set-ting the bit associatedwith a specific condition indicates that thecondition exists at the SS.Bit #0 SS already at maximum power. Bit #1SS already at minimum power. Bit #2 Sumof commanded timing adjustments istoo large.AAS410 = SS can receive broadcast messagesbroadcast1 = SS cannot receive broadcast messagescapability
TABLE 8Type(1Namebyte)LengthValue (variable-length)Downlink72This parameter is sent in response to theOperationalRNG-REQBurst ProfileRequested Downlink Burst Profileparameter.Byte 0: Specifies the leastrobust DIUC that may be used by the BSfor transmissions to the SS.Byte 1: Configuration Change Count valueof DCD defining the burst profileassociated with DIUC.SS MAC86SS MAC Address in MAC-48 formatAddressBasic CID92Basic CID assigned by BS at initialaccess.Primary102Primary Management CID assigned byManagementBS at initial access.CID
As described above, the mobile station forwards the ranging request message to the base station through the contention based uplink ranging field during network registration, and the base station transmits the ranging response message to the corresponding mobile station. This procedure is referred to as the initial ranging. If the mobile station performs ranging through the contention based uplink field, it may not receive the ranging response from the base station as collision with another mobile station which performs ranging occurs. If the mobile station forwards the ranging request and does not receive the ranging response from the base station for a certain time period, the corresponding mobile station retransmits the ranging request to the base station.
Furthermore, the base station may allocate the ranging field to a specific mobile station so that the mobile station may perform non-contention based ranging. If the mobile station performs handover to another base station in a normal action state, the mobile station should perform ranging with a corresponding base station. At this time, a service base station notifies a handover target base station (target BS) of handover of the mobile station, and the target base station can allocate the non-contention based ranging field using an uplink map information element (fast ranging IE) as shown in Table 9, wherein the non-contention based ranging field is ranging field where the corresponding mobile station can perform ranging during actual handover. Also, the mobile station can perform the ranging procedure with the handover target base station through the allocated ranging field.
TABLE 9SyntaxSizeNotesFast_ranging_IE {Extended UIUC 4 bits0x06Length 4 bitsLength = 0x0bHO ID indicator 1 bit0: MAC Address is present 1: HO ID is presentReserved 3 bitsShall be set to zeroif (HO ID indicator == 1) {HO ID 8 bitsReserved40 bitsShall be set to zero} else {MAC address48 bitsMS MAC address as provided on the RNG-REQ message on initial system entry}UIUC 4 bitsURIC ≠ 15. A four-bit code used to definethe type of uplink access and the burst type associatedwith that access.if (UIUC == 12) {OFDMA Symbol offset 8 bitsSubchannel offset 7 bitsNo. OFDMA Symbols 7 bitsNo. Subchannels 7 bitsRanging Method 2 bits0b00 - Initial ranging over two symbols0b01 - Initial ranging over four symbols0b10 - BW Request/Periodic ranging overone symbol0b11 - BW Request/Periodic ranging overthree symbolsReserved 1 bitShall be set to zero} else {Duration10 bitsIn OFDMA slots (see 8.4.3.1)Repetition coding 2 bits0b00 - No repetition codingindication0b01 - Repetition coding of 2 used0b10 - Repetition coding of 4 used0b11 - Repetition coding of 6 usedReserved20 bitsShall be set to zero}}
Hereinafter, a networking method using a relay station in a wireless metropolitan area network (wireless MAN) system will be described. FIG. 1 illustrates an example of networking using a relay station.
In IEEE 802.16, studies of multi-hop relay networking are being made to improve throughput by allowing the wireless MAN system to extend service coverage of the base station and provide higher signal quality to a mobile station in a shadow area.
In the IEEE 802.16, for relay networking, a frame structure of a physical layer has been improved, and a new protocol of a MAC layer has been additionally provided. According to main features of the IEEE 802.16, a tree structure where a final destination of a data path which is relayed is used as a base station is compatible with the existing point to multi-point (PMP) mode, and a relay mode and the PMP mode use the same frequency band or different frequencies adjacent to each other. Examples of the relay station include a fixed relay station, a nomadic relay station, and a mobile relay station.
FIG. 2 illustrates the operation of networking using a relay station according to the IEEE 802.16. Objects of the relay station can be categorized into service coverage extension of the base station and throughput improvement. The operation of the relay station can be defined differently as shown in FIG. 2 depending on the respective objects.
If the relay station is used for service coverage extension of the base station as shown in relay station type 1 of FIG. 2, a relay station 34 relays all of control messages, which are forwarded from a base station 31 or a mobile station 35, as well as user data exchanged between the mobile station 35 and the base station 31. If the relay station is used for throughput improvement as shown in relay station type 2 of FIG. 2, a relay station 32 relays only user data exchanged between a mobile station 33 and the base station 31 and allows a broadcast type control message of the base station 31 or an uplink control message of the mobile station 33 to be directly exchanged between the mobile station 33 and the base station 31. In this way, data relayed using the relay station may be delayed as compared with data directly exchanged between the mobile station and the base station. The relay station can increase overall throughput by providing excellent signal quality to the mobile station to which the data are relayed and relaying the data to the corresponding mobile station using a suitable coding rate and modulation mode.
In a wireless metropolitan area network (IEEE 802.21 wireless MAN) system, mobile multi-hop relay (MMR) related studies using a relay station are being actively made to achieve two objects such as cell area extension and throughput improvement. However, the detailed system structure and operation of MMR have not been defined until now.
In the aforementioned networking method using a relay station according to the related art, definition of ranging performed to allow the mobile station to perform registration to the base station or periodically maintain synchronization with the base station is very important. Nonetheless, definition of such ranging is unclear. In the MMR network, since the relay station exists between the mobile station and the base station of the existing system, ranging method of a channel between the relay station and the mobile station should be defined when communication through the relay station occurs.